This invention relates to semiconductor device comprising MOSFETs with higher and lower threshold levels and, more particularly, to growth of gate oxide films of multiple thickness on a semiconductor substrate, in accordance with the threshold levels and types of the MOSFETs.
The recent trend in CMOS-LSI circuit fabrication is to achieve high speed operation and low consumption power on the semiconductor device formed on one chip. To this end, MOSFETs on one chip are designed to operate with different threshold levels. Specifically, MOSFETs for low consumption power are designed to operate with a higher threshold level, while MOSFETs for high speed-driving are designed to operate with a lower threshold level.
It is known to the inventors that thicknesses of the gate oxide films of higher threshold MOSFETs are determined to be thicker than that of lower threshold MOSFETs, because of letting standby current of the higher threshold MOSFETs not depend on the gate-channel leakage current characteristics but on subthreshold characteristics of the higher threshold MOSFETs. If thicknesses of the gate oxide films of higher threshold MOSFETs are substantially equal to that of lower threshold MOSFETs, the gate-channel leakage current becomes larger than the subthreshold current (i.e., off-current) in the higher threshold MOSFETs, resulting in making the consumption power high.
Technologies for growth of oxide films of multiple thickness are disclosed, for example, in Japanese Unexamined Patent Publications (JP-A) Nos. H10-335656 and H11-162973. It is known to the inventors that the foregoing technologies are applicable for the fabrication of the semiconductor device having MOSFETs with different threshold levels. Specifically, the implantation treatment of fluorine ions into a predetermined region on a semiconductor substrate is carried out before a single thermal oxidation process, so that the single thermal oxidation process makes different thicknesses of the gate oxide films, because fluorine ions promote the thermal oxidation process. That is, the gate oxide film on the predetermined ion-implanted region is thicker than that of yet-ion-implanted region.